You need to have very specific rules about your channels here, especially if you are an RP oriented mud. I'm assuming that you don't have a lot of global channels and that your newbie channel is probably your most important one since it helps the new players get started. A newbie channel is for NEWBIE HELP. If you have to, restrict access to this channel for newbies and people who want to help newbies.
You can't really fight this. For some odd reason, some people really can't stand to see other people do better than them. Their first instinct is to accuse the player who does well of cheating, being "favored", having help, etc. The best thing you can do for this is to make everything transparent. If there are IMMs playing the game, people need to know who they are. If there are no IMMs playing, your rules for this need to be clearly stated and repeated.
If you make it clear that this is unacceptable behavior, then you can begin to use "time-out" punishments for people who do it. Ban them from OOC channels (if you have them, I'm assuming yes since these accusations are being made IC) for 24 hours on their first offense, etc. Be very careful that you only do this to the people who are repeat offenders. People will get your point sooner or later.
Discuss this behavior with your community. Explain why it is unacceptable and how it feeds itself. This is a cycle. Players who accuse others of cheating because they're powerful will get those players to quit, and sooner or later, they will be the ones accused.
This is a game design issue that really can't be solved easily. You can talk with the people in power about it, but be know that if you do so, you are impacting their gaming as much as they may be impacting the losing side's gaming. You can give encouragement for the people on the "winning side" to restart to the losing side, but keep in mind that your players may have a serious problem with this and see it as an advantage they never received. Honestly, you may have to just wait it out. The pendulum will often swing back the other way, but it may take quite some time to do that. Some players enjoy nothing more than being the underdog, and once their side starts "winning", they get bored. Of course, then you have players for which the opposite is true, and when the pendulum starts swinging, you may find that it swings rather quickly.
The only way to really prevent this is by game design choices. It sounds like your game is already established and in full swing, so I don't think this is an option.
